Process for the electrolytic formation of aluminum coatings on metallic surfaces in molten salt bath

ABSTRACT

PROCESS FOR COATING ELECTROLYTICALLY METALLIC SURFACES WITH ALUMINUM IN A MOLTEN SALT BATH CONTAINING ALUMINUM CHLORIDE. THE METALLIC SURFACE IS COATED WITH AN INORGANIC AQUEOUS SOLUTION AND THEN IS ELECTROLYTICALLY COATED IN THE BATH. THE PROCESS RESULTS IN COTAINGS OF SUPERIOR SMOOTHNESS AND BONDING STRENGTH BETWEEN THE SURFACES.

United States Patent '0 PROCESS FOR THE ELECTROLYTIC FORMATION OFALUMINUM COATINGS ON METALLIC SUR- FACES IN MOLTEN SALT BATH AkiraMiyata, Chikayoshi Tomita, and Akio Suzuki, Tokyo, and Hideyo Okubo andMasahiko Nagakuni, Kanagawa-ken, Japan, assignors to Nippon KokanKabushiki Kaisha, Tokyo, Japan No Drawing. Application Mar. 24, 1969,Ser. No. 828,049,

now Patent No. 3,699,012, which is a division of application Ser. No.507,034, Nov. 9, 1965, now Patent No. 3,480,521. Divided and thisapplication July 6, 1970, Ser. No. 60,994

Claims priority, application Japan, Nov. 13, 1964, 39/63,827; Feb. 24,1965, 40/10,330, 40/ 10,331 Int. Cl. C23b 5/22, 5/48 US. Cl. 204-29 2Claims ABSTRACT OF THE DISCLOSURE Process for coating electrolyticallymetallic surfaces with aluminum in a molten salt bath containingaluminum chloride. The metallic surface is coated with an inorganicaqueous solution and then is electrolytically coated in the bath. Theprocess results in coatings of superior smoothness and bonding strengthbetween the surfaces.

RELATED APPLICATION This application is a division of application Ser.No. 828,049, filed Mar. 24, 1969, and now Pat. No. 3,699,- 012, which,in turn, is a division of application Ser. No. 507,034, filed Nov. 9,1965, and now Pat. No. 3,480,521.

This invention relates to a process for coating steel, copper and othermetallic surfaces with aluminum as protecting films by means of moltensalt electrolysis in a fused bath containing aluminum halide and thelike.

It is commonly known to those skilled in the art that various andconsiderable drawbacks are encountered for carrying out this kind ofelectrolytic process. Among others, a speedy and unavoidabledeterioration of the electrolyte was encountered in the course ofrepeated electrolysis. This kind of deterioration will invite overall orlocalized loose deposit of aluminum coating on the metallic surface tobe protected, prevent favorable higher cathodic current density frombeing adopted or result even in formation of trees or slug-likenon-tight deposits on the metallic surface. In extreme cases, thecoating will represent powder-like appearance and is highly liable to bescaled-off even in the course of the water cleaning stage which isconventionally carried out in direct succession to the electrolyticcoating process.

For avoiding these drawbacks, addition of various salts such as those oflead, cadmium, chromium and/or the like, has hitherto been proposed andemployed. Inclusion of lead and other metals, up to 0.4%, in thealuminum coating, resulted however in a lowered anticorrosiveperformance, an accelerated scale-off tendency, and a rough touch of thecoated aluminum film.

It is therefore the main object of the invention to pro vide an improvedprocess for electrolytically coating steel and other metallic surfaceswith aluminum in a fused salt chloride bath, providing and assuring asubstantially extended durable life of the bath adapted for theelectrolysis.

Another object of the invention is to provide a process of the abovekind, capable of obtaining aluminum-coated metallic products havingsuperior bond of the coating to the metallic stock electrolyticallytreated.

It is still another object to provide a process of the kind abovereferred to, capable of obtaining aluminumcoated metallic productshaving superior surface conditions of the coating thus produced;

3,778,356 Patented Dec. 11, 1973 It is a further object to provide amolten salt electrolytic process easy to operate at a high efliciency.

These and further objects, features and advantages of the invention willappear more specifically and clearly as the description proceeds.

In the improved process as proposed by the present invention, hydrogenions are suplied in either or both positive and negative sense while theprocess proceeds. There are numerous Ways for carrying out the process.As a representative way to positively supply the hydrogen ions,electrochemically ionized hydrogen, for instance, by contact withplatinum black and/or hydrogen chloride gas may be fed directly to theelectrolytic bath.

Superior results obtainable by the process of this invention can beattributed to the fact that the aluminum as deposited is loaded withhydrogen ion in a direct or an indirect manner.

An additional or alternative way for the supply of hydrogen ions is toprovide prior to the electrolysis a very thin aqueous film on themetallic surface to be aluminumcoated, and only then subject the stockto the coating electrolysis afterwards. The precise mechanism, whyhydrogen ions can be supplied by adopting such measure, is not clear tous at the present moment. It can be assumed with a certain degree ofprobability that the aqueous content of the preliminarily formed andmaintained film will react with A101 contained in the molten salt bath:

In this Way, hydrogen ions may be supplied to the electrolyticallyreacting zone in the course of the coating process. As the wettingsolution, aqueous acetone solution of any desired concentration maypreferably be used. According to our experiments, superior results maybe obtained when the aqueous wetting solution contains, solely or incombination, inorganic and/or organic subtsances or chemicals which havea high aflinity to water. For this purpose, methyland/or ethyl alcohol,ethylene glycol, glycerine, lactic acid and/or formic acid may beemployed.

Starch, sugar, casein, egg albumen, gelatin, water glass or the likeorganic and inorganic substances may be utilized or the same purpose, inthe form of syrup or paste.

Various inorganic compounds such as sodium chloride, magnesium chloride,potassium chloride, calcium sulfate, magnesium perchlorate, causticsoda, calcium chloride, aluminum chloride, barium chloride, zincchloride, chromium chloride, titanium tetrachloride, and the like mayalso be used in the form of an aqueous or alcoholic solution for thesame purpose. When necessary, conventional surface active agents, forinstance, soap, various known organic derivatives such as sodium saltsof high molecular alkyl sulfates, or sulphonates may be used in the formof a dilute aqueous or alcohol solution for the purpose of thisinvention.

Even when these substances are dried up, they will contain a slightamount of water, thus in effect, in the form of an aqueous film whichcan be utilized according to this invention in the aforementionedmanner.

In the art of molten salt electrolysis for the formation of aluminumcoatings, an aluminum mass is frequently submerged in the fused bath soas to act as an anodic electrode, and to replenish the consumed quantityof the aluminum component in the bath as the electrolytic processproceeds. The mass dissolves out gradually in the v molten hot bathregardless of conducting the electrolytic As the unstable metal aluminumappears in the bath in state in the presence of hydrogen ions assupplied according to the main feature of this invention, as in suchcase metallic aluminum is liable to be oxidized in the presence ofhydrogen ions when hydrochloric acid is supplied for contact therewith.In this way, the aforementioned drawbacks adversely affecting the idealelectrolytic formation of aluminum coating may elfectively be obviatedrelying upon the novel teaching of the present invention.

In the inventive process, the supply of hydrogen ions to theelectrolytically reaction zone can be carried out in negative sense inthe following way:

More specifically, the metal stock is preparatorily and cathodicallysubjected to an electrolytic treatment in the presence of an acidsolution containing cations of those metals which have higher hydrogenovervoltage than that for aluminum, such as Pb, Sn, Ti, Zn, Bi, Cr, Cdand the like. By this preparatory treatment, these metals are depositedon the stock and carried by the stock to the electrolytically reactingzone in the next succeeding electrolytic aluminum coating process. Whenthe latter process is carried out in the presence of these metals,hydrogen ions contained in the fused salt bath, especially thoseexisting in close proximity of the electrolytically reacting zone, morespecifically in the neighborhood of the metallic surface to be subjectedto the aluminum coating process, are retarded in the transformation intohydrogen molecules. Although this retarding period is short, it isenough to carry out the electrolytic aluminum coating process underfavorable conditions. In this way, hydrogen ions may be supplied in anegative sense to the reacting zone, without relying upon any supplysource outside of the fused salt bath.

EXAMPLE 1 Using a molten bath, 200 cc., comprising 60 mole percent ofaluminum chloride and 40 mole percent of sodium chloride, a steel stockx x 0.2 mm.) was electrolytically treated as cathode in a conventionalmanner. Below the steel stock a length of approximately cm. of coiledplatinum wire (0.5 mm. thick and 10 mm. coil diameter) was held as anodeunder tension at a distance of 2 cm. while being submerged in the bath.The cathodic current density was 2 amp./dm. and the electrolysis wascontinued for 10 minutes at 160 C. In the course of the electrolysis,hydrogen gas was supplied in the form of small bubbles at a rate of 40cc./min. to the bath from below so as to contact the coiled anode. Inthis way, an aluminum coating having a thickness of 3 microns andsuperior surface conditions and an excellent bonding performance couldbe formed on both surfaces of the sheet stock.

EXAMPLE 2 For comparison, argon gas again in the form of small bubbleswas introduced instead of hydrogen under the same operating conditions,and at the same feeding rate as before and throughout the wholeelectrolytic period which was again 10 minutes. An aluminum coating wasproduced on the steel stock, with no appreciable improvement in thequality of the coating. Although agitation of the bath was utilized, noappreciable gain in the desired results were achieved, because of thelack of hydrogen ion.

EXAMPLE 3 1180 grams of aluminum chloride and 320 grams of sodiumchloride were mixed together thoroughly, heated to melting temperature,charged in a glass vessel and kept at a temperature of 160 C., whereuponthe vessel was sealingly closed off from the ambient atmosphere. Beforethe bath was sealed, a sheet of aluminum plate (50 x 150 x 0.2 mm.) wasarranged in the melt as anode and a piece of steel sheet having the samedimensions was provided as cathode. Current was conducted thereto at acathodic density of 2 amp./dm. at 0.4 volt for 10 minutes.

Current Weight of efIi- Time Al-coil, ciency, lapsed, hrs grams State ofAl-eoating percent None 44. 9408 Smooth, tight and well-bonded.... 83 2441.5588 do 69 40. 4352 Formation of trees, partially 68 141 38. 9280Formation of trees, overall 40 261 37. 1200 Pulverized state, overall 15Even with a slight formation of trees, the deposited aluminum was liableto be scaled off in the course of the succeeding water cleaning step,thus the apparent current efficiency actually became still lower and ina suddenly decreasing manner. When the anodic current efficiency wasmeasured relative to the dissolved-out quantity of aluminum, it amountedfrequently to more than thus the aluminum content of the bath increasedwith the duration of the electrolytic treatment. It was observed thataluminum once deposited on the cathode dissolved out again, as evidencedby reduction of weight of the aluminum wire.

On the other hand, likewise in the above experiment, when the aluminumwire coil was kept in its submerged state in the molten bath at 160 C.for 30 days, a further dissolving-out of the aluminum could not beobserved. When the bath temperature was reduced to C. air bubblesentering the bath were observed to be accompanied o-n the marginalsurface thereof by separated aluminum in the form of thin scales incrystalline state, which is also a sign of the presence of the saidsupenfluous solute of aluminum.

EXAMPLE 4 Using a molten bath comprising 62 mole percent aluminumchloride and 38 mole percent sodium chloride, but without any additionof conventional metal mist suppressing agent such as potassium chlorideso as to clearly observe the aforementioned adverse effects by thepresence of superfluous aluminum solute as set forth in the foregoingExample 3, the electrolytic treatment was carried out at C. for about 10minutes with a cathodic current density of 2 amp./dm. taking an aluminumplate as anode and a steel sheet as cathode, having the same dimensionsas given above, respectively, the voltage being 0.3-0.4 volt and thetreatment being repeatedly performed from several times to about 20times per day and for an extended time period, such as 10 months. On thefirst day, the current efliciency amounted to 85-90% which decreasedhowever to 62- 70% on the third day. On the tenth clay, efiiciencydecreased to as low as l0-20%. This made it very diflicult to carry outthe electrolytic coating process with satisfactory quality of thecoating produced.

When, however, gaseous hydrogen chloride was supplied to the molten bathon the third day at a rate of 80 cc./min. per one liter of the bathmaterial and for about an hour, so as to provide hydrogen ions to theelectrolytically reacting zone, the current efiiciency was restored to80% from the preceding value of 62-70%. When the duration of the supplyof gaseous hydrogen chloride was further extended for 1.5 hours, theefiiciency rose to 85% which high value could be maintained for as longas six months by adopting the blowing-in operation of hydrogen chloridewhile continuing the electrolytic coating operation.

5 EXAMPLE 5 A steel sheet stock, having the same dimensions as set forthin Example 3, was degreased, pickled and well water-cleaned, and wasthen used as cathode in an aqueous solution containing 1% of HCl and0.005% of PbCl at a current density of 1 ampJdm. for about 10 seconds,using a carbon electrode of equal dimensions as above, as anode. Thestock was then water-cleaned to a satisfying degree. In this case, thestock should preferably be subjected to the influence of mechanicalvibration preferably at several to about 100 cycles per second.

By water cleaning, soluble salts adhering to the steel surface weresubstantially removed therefrom, and then the stock was dried.

Should the treating liquid contain a higher concentration of PbCl thanthe above-specified value of 0.005%, the treated steel surfacerepresented, to a slight degree, a brown to gray coloring, but stillexhibited the natural glazing color of steel.

On the other hand, when PbCl-jcontent was lower than 0.00l%, theappearance of the treated steel surface, was substantially unchanged.

The thus treated stock was then subjected to a conventional electrolytictreatment in .a molten salt bath comprising 5 8 mole percent of AlCl and42 mole percent of NaCl accordinng to the conventional teachnique forthe formation of aluminum coating, providing thus a smooth, glazing andwell-bonded aluminum film on the stock and capable of being subjected,without any cracks in the coating, to a subsequent mechanical workingfor the fabrication of finished products. The thickness of the aluminumcould be adjusted with ease to a value thicker than microns whichconstitutes a considerable advance in the art.

EXAMPLE 6 In this experiment, the content of PbCl; was adjusted to0.005% and the concentration of HCl was 3 N, the current density being 1amp. dm-F. The preliminary treatment was carried out for about 30seconds, further conditions for the preliminary step being the same asdescribed in Example 5.

The pretreated stock was further treated electrolytically with a currentdensity of 2 amp./dm. as in the preceding Example 5, thus providing analuminum coating, 17.4 thick, with a current efiiciency of 88%. Thecoating was beautiful, of metal glazing tone, and wellbonded withsuperior quality.

EXAMPLE 7 In this experiment, the content of PbCl was adjusted to0.0006% and the concentration of HCl was 0.3 N, the current densitybeing 1 amp./dm. The preliminary treatment was carried out for about 30seconds, further conditions for the preliminary step being the same asdescribed in Example 5.

The thus treated test piece was further treated electrolytically with acurrent density of l amp./dm. in a bath which had become highlydeteriorated so that only a degreased and pickled stock could not havebeen treated therewith effectively, other conditions being similar tothose as employed in Example 5. The thus formed aluminum coating wasbeautiful in its metal glazing tone, and well-bonded with superiorquality.

EXAMPLE 8 In this case, theaqueous preliminary treating bath contained0.1% of bismuth oxide and 1.0% of perchloric acid. The treatmentwascarried out with a current density of 1 amp./dm. for about 30 seconds.

Then, the treated stock was subjected further to an electrolytictreatment while using the same molten salt bath as was set forth inExample 7, and using a current density of 1 amp./dm. Superior results inthe formed aluminum coating were obtained. In this case, however,

the coating thus produced should be exposed to the open atmosphere atleast once in the course of the next succeeding water-cleaning step, soas to convert the produced coating into an inactivated one.

EXAMPLE 9 In this case, an aqueous solution of hydrochloric acid havinga concentration of 1 N and containing 0.05% of stannous chloride wasused as preliminary treating liquor. The treatment was carried out witha current density of 1 amp./dm. for about 30 seconds.

Then, the treated stock was subjected further to an electrolytictreatment while using the same molten bath as was set forth in Example7, and with a current density of l amp./dm. again. Aluminum film wascoated on the stock with superior results.

EXAMPLE 10 In this case, the preliminary treating aqueous liquorcontained 0.5% of lead acetate. The treatment was carried out with acurrent density of 0.3 amp./dm. for about 2 minutes.

The thus treated stock was then subjected to the second treatment, asbefore, for coating it with aluminum, yet while using same bathcompositions as set forth in Example 6. The coating was slightlyinferior in its characteristics, yet having a superior metallic glaze.

EXAMPLE 11 The preliminary treatment, was carried out with use of anaqueous 2 N hydrochloric acid solution containing 0.5% of titaniumchloride at a current density of 20 amp./dm.

Then, the thus treated stock was further subjected to an electrolyticcoating treatment While using the same molten salt bath as set forth inExample 7. The current density was 1 amp./dm. as before. The thusobtained aluminum coating had a superior metal glaze, providing inaddition, superior tightness, smoothness and bond.

EXAMPLE 12 A steel sheet stock was preliminarily coated with a thin iilmof acetone of purity, by dipping the stock there- 1n.

Then, the treated stock was subjected to an ordinary electrolytictreatment, using a used-up molten bath comprising 58 mole percent AlCland 42 mole percent NaCl, and kept at C. This bath had so deterioratedas to be considered unusable for conventional operation. Theelectrolyiss was carried out with a current density of 1.5 amp./drn. forabout 10 minutes. The aluminum coating which was obtained was completelysmooth, tight, wellbonded and highly durable to scratching.

The acetone was replaced in succession 'by methyl alcohol, ethyl alcoholof 96% purity; ethylene glycol, 95%; glycerin, 95 lactic acid, 87%; andformic acid, 87%. The results were similar.

EXAMPLE 13 An aqueous syrup (concentration: 1%) made of starch was usedin place of the acetone used in Example 12, and the treating conditionswere the same as set forth in that example. Similar superior resultswere obtained.

When the starch was replaced in succession by sugar (concentration: 10%casein (2%), egg albumen (1% gelatin (1%) and Water glass (0.5% similarsuperior results were obtained.

EXAMPLE 14 A dilute aqueous solution of common salt (concentration: 5%)was used and other treating conditions were the same as set forth inExample 13. Similar superior aluminum coatings were obtained.

When the common salt was replaced in succession by magnesium chloride 5%potassium chloride (5% calcium sulfate (0.2%), magnesium perchlorate(0.5%),

caustic soda (1%), calcium chloride (5%), aluminum chloride (5% bariumchloride (2% zinc chloride (2% and chromium chloride (1% substantiallythe same superior results were obtained.

The dipped metal stock was dried at normal temperature and then left inthe open air so as to absorb a certain quantity of water content fromthe ambient air, so as to form a thin aqueous film on the stock surface.

When any one of the above chemicals was replaced by a small amount ofconventional surface active agents, such as an alkylated sodiumbenzenesulfonate, or added in addition, similar results were obtained.

EXAMPLE A combination of two or more of the substance referred to in theforegoing Examples 12-14 could be employed with equal results when otheroperating conditions were similar to those set forth therein.

For instance, ethyl alcohol or an aqueous solution thereof was addedwith a small amount of TiCl, and coated in a thin film on a steel orother metallic sheet stock and the latter was treated under similarconditions as before, the resulting aluminum coating was highly superiordue to the combined action of both chemicals. In this case, the stockafter being dipped, was treated while in its wet condition. Even whenthe preliminarily treated stock was dried in a hot air drier and thentreated as before, the nature and conditions of the obtained aluminumcoating were substantially the same as before.

Although steel stocks have only been described hereinbefore, othervarious metallic stocks such as copper and the like may equally betreated by the inventive process. This will apply also to other metalsthan steel.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic aspects of this invention, and therefore,such adaptations should and are intended to be comprehended within themeaning and range of equivalence of the following claims.

What is claimed is:

1. In the process for electrolytically coating a metallic surface withaluminum in a molten salt bath containing aluminum chloride and whereinan aluminum metal member is used as an anode and the metallic surface tobe coated as cathode, the improvement which comprises: preliminarilycoating said metallic surface with an inorganic aqueous solution andthereafter electrolytically coating said preliminarily coated metallicsurface in said bath, wherein the inorganic material of said solution isselected from the group consisting of magnesium chloride, potassiumchloride, calcium sulfate, magnesium perchlorate, calcium chloride,aluminum chloride, barium chloride, zinc chloride, chromium chloride,titanium tetrachloride, lead chloride, lead acetate and stannouschloride.

2. Process of claim 1, wherein the inorganic material of said solutionis hygroscopic.

References Cited UNITED STATES PATENTS 3,136,709 6/ 1964 Smith et a1.204-39 3,480,521 11/1969 Miyata et al. 20439 3,259,557 7/ 1966 Smith eta1. 20439 2,752,303 6/1956 Cooper 204-246 OTHER REFERENCES The SurfaceTreatment and Finishing of A1, by Wernick et al., 3rd ed., 1964, p. 168.

Metal Finishing Guidebook Directory, 36th ed., 1968. p. 536.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl.X.R. 20439

